Nauclea officinalis: A Chinese medicinal herb with phytochemical, biological, and pharmacological effects

Nauclea officinalis (N. officinalis), a medicinal plant of the genus Nauclea in the family Rubiaceae, is used in the treatment of fever, pneumonia, pharyngolaryngitis, and enteritis in China. Extracts of N. officinalis include alkaloids, phenolic acids, pentacyclic triterpenoids, and flavonoids, which exert all kinds of pharmacological effects, for instance anti-inflammatory, anti-tumor, antibacterial, and antiviral and therefore show good effectiveness. To gain a comprehensive and deep understanding, the medicinal chemistry and chemical biology of N. officinalis are summarized in this review to provide a theoretical basis. The pharmacological effects were reviewed to provide evidence or insights into potential opportunities for further studies and medicinal exploitation of N. officinalis.


Introduction
Nauclea officinalis (N. officinalis), also known as Dan Mu, is a traditional herbal medicine distributed in Hainan, Yunnan, and Guangxi in China [1]. N. officinalis belong to the genus Nauclea in the family Rubiaceae [2,3]. A large majority of these are woody trees and shrubs that are primarily found in tropical regions [4]. In Li folk medicine, N. officinalis has been used to treat inflammatory and infectious diseases including diarrhea, pneumonia, and enteritis [5][6][7].
Alkaloids are the main secondary metabolites found in N. officinalis, which are heterocyclic molecules containing at least one nitrogen atom in their structure [17]. As the main classification of alkaloids, indole alkaloids, which have antibacterial, antimalarial, antifungal, antiparasitic, anti-inflammatory, antiviral, antineoplastic, anti-acethylcolinesterase, and anti-buthyrylcolinesterase properties [18,19], are chemical components of N. officinalis. For example, strictosamide (STR) and vincosamide are the main active components of N. officinalis indole alkaloids [20]. STR is the most abundant constituent of indole alkaloids and has a variety of pharmacological effects, such as anti-inflammatory properties, antipyretic properties, and antiviral properties [21]. Vincosamide is an isomer of STR that exhibits anti-tumor [22], antiinflammatory [23], and other properties. According to pharmacokinetic studies, these two compounds are readily absorbed into the plasma and exhibit good pharmacological effects.
The compounds of quinoline alkaloids have N-based heterocyclic aromatic and have a broad range of biological activities including anti-tumor, antiparasitic, antibacterial, cardioprotective, antiviral, anti-inflammatory, antioxidant, and antitussive activities [24,25]. Triterpenoids make up this diverse class of natural plant products, which contain three terpene units and are effective against tumors, inflammation, and viruses [26]. Phenolic acids are phenolic compounds that are resonance stabilized and possess a phenol moiety. As a result, antioxidant properties are achieved through radical scavenging mechanisms that lead to H-atom donation. Furthermore, phenolic acids have health protective effects, for instance anti-microbial, anti-tumor, anti-inflammatory, and antimutagenic effects [27]. These findings indicate the potential medicinal value of N. officinalis as a Chinese herbal medicine.
Our review summarizes phytochemical, bioactive, and pharmacological effects of N. officinalis, providing useful information for its development and utilization.

Phytochemistry and biological activity of N. officinalis
Numerous types of chemical constituents have been identified in N. officinalis, including alkaloids, pentacyclic triterpenoids, and phenolic acids, according to phytochemical analysis (Table 1). Alkaloids are the characteristic components of N. officinalis and its main active ingredients.

Alkaloids
Alkaloids are the most widely reported class of compounds found in N. officinalis. The alkaloids in N. officinalis are mainly indole alkaloids; however, a few quinoline alkaloids are also present. Studies have shown that certain indole alkaloids exhibit anti-inflammatory, antibacterial, and antiplasmodial activity.

Indole alkaloids
The indole alkaloids in N. officinalis have the following characteristics: the structural skeleton generally consists of five six-membered rings; indoles (A/B ring) and tetrahydropyridines (C ring) form the terahydro-βcarboline parent ring, the D ring is a saturated or unsaturated lactam ring, the E ring can be an azapyridine or oxatetrahydrofuran ring, and some compounds have an open or non-existent E ring [12].
Phytochemicals such as nucleactonin A and B are synthesized from N. officinalis bark and wood [2]. Only naucleactonin A has been shown to exhibit anti-inflammatory activity [37].

Quinoline alkaloids
Another class of N-based heterocyclic compounds is quinazoline alkaloids. There are about 150 quinazoline alkaloids found in plants, animals, and microorganisms that occur naturally. Several them are genetically descended from anthranilic acid [24]. The quinolone alkaloids pumiloside and 3-epi-pumiloside, extracted from the stems, are among the main active components of N. officinalis [43], are present in high amounts, and exhibit various effects, such as anti-inflammatory, antibacterial, and anti-tumor activities [7,44,45].

In vivo experiment
N. officinalis is a strong modulator of inflammatory immune responses and suppresses infection and development during the initial stages of infection and inflammation. Asthma mice can be regulated by N. officinalis affecting the secretion of cytokines, such as interferon gamma (IFN-γ), interleukin (IL) -10, IL-5, IL-4, and IL-2, and the infiltration of inflammatory cells in the airway [60]. N. officinalis leaves purified by alcohol extraction-macroporous resin inhibited systemic and local inflammatory responses in rats with acute pharyngitis [61]. Acute inflammation occurs at the beginning, N. officinalis extract tablets inhibited exudation, swelling, and late granulation tissue formation in animals with an inflammatory response model. In addition, N. officinalis injection inhibited inflammatory cell infiltration in asthmatic mice, thereby improving bronchial asthma [62]. STR is the main representative constituent of N. officinalis and contributes significantly to ameliorate inflammation by downregulating the expression of pro-inflammatory cytokines, including tumor necrosis factor (TNF-α), IL-1β, and IL-6, and inhibiting the NF-κB signaling pathway [63]. A significant reduction in the edema caused by terephthalic acid (TPA) was observed at 20 and 40 mg/kg of STR, and a significant reduction in mouse peritoneal vascular permeability in response to acetic acid. Furthermore, STR significantly reduced leukocyte numbers within the mouse peritoneum induced by sodium carboxymethyl cellulose (CMC-Na). In 20 mg and 40 mg/kg doses of STR, pain latency was markedly prolonged, and the number of writhes was decreased when 40 mg/kg was used [14].

Anti-microbial effect
He et al. [10] observed the sensitivity of Escherichia coli to N. officinalis decoction through an in vitro antibacterial test. The results showed that N. officinalis has a bacteriostatic effect on clinically isolated urinary Escherichia coli. Xu et al. [65] found that the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of N. officinalis extract against Staphylococcus aureus ranged 1.56-3.13% and 1.56-25%, respectively. Staphylococcus aureus has good antibacterial and bactericidal activities against both clinically resistant and sensitive strains, and its MIC to bacteria is close to MBC, suggesting that its antibacterial activity may be related to its direct bactericidal activity. Su et al. [66] detected the antibacterial activity of volatile components using the method of paper-disk diffusion. Results revealed that the volatile components of N. officinalis leaves and stems had strong antibacterial effects in Methicillin-resistant Staphylococcus aureus, Escherichia coli, Bacillus subtilis, and Proteus. It has also been reported that N. officinalis has significant bactericidal and antibacterial effects on bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, Streptococcus haemolyticus, and Shigella dysenteriae. Jiang et al. [67] showed that the combined antibacterial effect of N. officinalis extract and penicillin showed synergistic action and had an efficient inhibitory effect on S. aureus.

Antiviral effect
Children were treated with N. officinalis and ribavirin injections for acute upper respiratory tract infections. The results showed that N. officinalis injection inhibited bacterial and viral protein synthesis, and metabolism of folic acid is blocked. The clinical effect of N. officinalis is more significant than that of ribavirin injection and has the advantages of broad-spectrum antibacterial, anti-virus, and less susceptibility to drug resistance [61]. N. officinalis stems and leaves are rich in monoterpene indole alkaloids, which exhibit significant anti-HIV-1 activities, with IC 50 s ranging from 0.06-2.08 µM. N. officinalis may be a valuable source of indole alkaloids with significant anti-HIV-1 activities that could be developed into new anti-HIV agents. [37]. The anti-HIV-1 activities of Naucleaoffines A and B, naucleactonin A, nauclefidine, 3,14-dihydroangustine, 3,14,18,19-tetrahydroangustine, angustine, and naucletine were, with EC 50 ranged between 0.06 and 2.08 μM [37]. Biological studies confirmed this activity, by measuring the IC 50 values of 25.68 g/mL for influenza A virus and 12.50 g/mL for respiratory syncytial virus [15].
In the anticholinesterase activity test, angustidine, nauclefine, angustine, angustoline, and andharmane showed higher butyrylcholinesterase inhibitory potency. Angustidine is known to be one of the greatest inhibitors of acetylcholinesterase and butyrylcholinesterase, which forms hydrogen bonds with Ser198 and His438 [40].
In addition, different concentrations of STR decreased Mg 2+ -ATPase activity in the brain and kidneys in vivo and in vitro. STR also increased Na + /K + -ATPase activity in the brain in vivo. These results may account for the effect of N. officinalis infusions on hypertension [69]. Although there are many studies to explore the pharmacological effects of N. officinalis (Table 2), there is still a lack of in-depth research on its mechanism of action, which also indicates that N. officinalis and its active ingredients have great research and development potential.

Conclusions
As one of the rare wild plant species under key protection in China, N. officinalis is also an important medicinal plant. The dry trunk, branches, bark, leaves, and roots of N. officinalis can be medicated, and has the effect of clearing heat and detoxification, reducing swelling and relieving pain. The chemical composition of N. officinalis mainly includes alkaloids, pentacyclic triterpenes and their saponin compounds, phenolic acids, flavonoids, amino acids, and various trace elements. In recent years, from the consideration of expanding the source of drugs, people have paid more and more attention to composition and biological activity of N. officinalis. The content of total flavonoids, total flavone glycosides and alkaloids in N. officinalis is high, especially the content of alkaloids is as high as 3.46%, which has high medicinal value and good development potential [70].
A variety of pharmacological effects are associated with N. officinalis, including anti-inflammatory, anticancer, anti-microbial, and antivirus ( Fig. 1). In clinic, N. officinalis extract syrup was used to treat acute tonsillitis, pediatric viral influenza, and lower respiratory tract infection with good results [71][72][73], but more rigorous researches need to be conducted. In some basic research, although a series of anti-inflammatory, anti-tumor, and anti-microbial studies have been conducted for N. officinalis and its extracts, these studies only briefly showed their pharmacological effects. In addition, several studies have used extracts of N. officinalis, but the specific active ingredient is unclear, which is not conducive to the study of the mechanism of N. officinalis. Therefore, exploring the in-depth mechanisms of N. officinalis, its extracts, and active ingredients is urgent and may help identify new drugs and targets of drug action. In recent years, with the development and utilization of N. officinalis herbs, resources have become increasingly important and the full and rational exploitation of N. officinalis resources has become the focus of future research. Meanwhile, developing new preparations or discovering more medicinal value is also an important issue for the development of N. officinalis.